This invention relates to compositions which contain organoborane amine complexes useful for initiating polymerization of compounds by free radical means. In another embodiment, this invention relates to polymerizable compositions comprising compounds containing moieties capable of free radical polymerization and organoborane amine complex initiators of the invention. In yet another embodiment the invention relates to a method of polymerizing compounds containing moieties capable of free radical polymerization.
In many practical situations in which compounds are subjected to polymerization or where adhesives are used, it is desirable to have polymerizable compositions and adhesive compositions which can cure on demand. Cure on demand means that the polymerization can be initiated when desired. A significant problem with cure on demand compositions is the stability of the compositions. Many such compositions will begin curing at, or near, ambient temperature or will partially cure at ambient temperature resulting in an increased viscosity causing difficulties in handling and reduced functionality of the polymerizable composition or adhesive composition.
Low surface energy olefins such as polyethylene, polypropylene and polytetrafluroethylene have a variety of attractive properties in a variety of uses, such as for toys, automobile parts, furniture applications and the like. Because of the low surface energy of these plastic materials, it is very difficult to find adhesive compositions which bond to these materials. The commercially available adhesives which are used for these plastics require time consuming or extensive pretreatment of the surface before the adhesive will bond to the surface. Such pretreatments include corona treatment, flame treatment and the like. The requirement for extensive pretreatment of the surface results in significant limitations to the designers of automobile components, toys, furniture and the like. What are needed are adhesive compositions which are capable of bonding to low surface energy substrates, and bonding low surface energy substrates to other substrates, without the need for extensive or costly pretreatment.
Mottus et al., U.S. Pat. No. 3,275,611 (incorporated herein by reference) discloses a process for polymerizing olefinic compounds with a catalyst comprising an organoborane compound, a peroxygen compound and an amine. It is disclosed that the organoborane compound and amine may be added to the reaction mixture separately or as a preformed complex, and that the complex is preferred. The presence of the amine in the complex reduces the pyrophoricity of the organoborane in air. Among the amine complexing agents disclosed are pyridine, aniline, toluidine, dimethylbenzylamine, and nicotine. Many of the complexes disclosed in Mottus are pyrophoric at all amine to boron atom ratios. In addition, many of the amine complexes do not display significant adhesive properties when applied to low surface energy substrates.
A series of patents issued to Skoultchi (U.S. Pat. Nos. 5,106,928, 5,143,884, 5,286,821, 5,310,835 and 5,376,746 (all incorporated herein by reference)) disclose a two-part initiator system that is useful in acrylic adhesive compositions. The first part of the two-part system includes a stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde. The organoborane compound of the complex has three ligands which can be selected from C1-10 alkyl groups or phenyl groups. Useful amines disclosed include octylamine, 1,6-diaminohexane, diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine, 1,3-propylene diamine, and 1,2-propylene diamine. The adhesive compositions are disclosed to be useful in structural and semi-structural adhesive applications, such as speaker magnets, metal to metal bonding, automotive glass to metal bonding, glass to glass bonding, circuit board component bonding, bonding select plastics to metal, glass to wood, etc. and for electric motor magnets.
Zharov et al. discloses in a series of U.S. patent Nos. (5,539,070; 5,690,780; and 5,691,065 (all incorporated herein by reference)) polymerizable acrylic compositions which are particularly useful as adhesives wherein organoborane amine complexes are used to initiate cure. The organoboranes used have three ligands attached to the borane atom which are selected from C1-10 alkyl groups and phenyl. The amine is an alkanol amine, a linear alkyl diamine where the first amine group can be a primary or secondary amine and the second amine is a primary amine or an aromatic substituted alkyl amine. It is disclosed that these complexes are good for initiating polymerization of an adhesive which bonds to low surface energy substrates.
Pocius in a series of patents (U.S. Pat. No. 5,616,796; U.S. Pat. No. 5,621,143; U.S. Pat. No. 5,681,910; U.S. Pat. No. 5,686,544; U.S. Pat. No. 5,718,977; and U.S. Pat. No. 5,795,657(all incorporated herein by reference) disclose amine organoborane complexes with a variety of amines such as polyoxyalkylene polyamines and polyamines which are the reaction product of diprimary amines and compound having at least two groups which react with a primary amine. Pocius U.S. Pat. No. 5,686,544 discloses a composition comprising an organoborane polyamine complex, polyol and an isocyanate decomplexing agent. It is disclosed that the primary amine to boron ratio in the complex is between about 4:1 and 1:1 and most preferred at about 1:1.
Many of the complexes disclosed in the Zharov, Skoultchi and Pocius Patents are not stable in compositions containing olefinic unsaturation at, or near, ambient temperatures and thus the complexes disassociate and induce polymerization at, or near, ambient temperature with time. This instability at, or near, ambient temperature can result in polymerization before desired and can result in compositions which are unsuitable for the desired use. Further, the compositions generally are two part compositions in which one side is a resin side and the other side is a hardener. One side (hardener) contains the organoborane complex and the other contains the decomplexing agent. In most cases the volume ratio of the two parts is significantly different, that is greater, than 4:1, often greater than 10:1. The problem is that most commercial equipment designed to dispense two part compositions utilizes ratios of 4:1 or less. In order to get these compositions to work in such equipment resin or nonreactive ingredients are added to one side or the other to get a suitable volumetric ratio. The problem is that if resin is added to the side containing the complex the mixture is unstable and could begin to cure at ambient temperatures and render the composition useless. If excessive inert materials are added to the hardener side the inert ingredient can act as a plasticizer or create a weak continuous phase and negatively impact the properties of the polymerized composition.
After polymerization many of the compositions of the prior art demonstrate excellent stability, strength and adhesion at or near ambient temperatures, but at elevated temperatures undergo loss of strength and adhesion at significantly elevated temperatures. This limits the environments where substrates bonded using these adhesives can be used.
Therefore, there is a need for initiator systems for free radical polymerization which are safe to handle, not pyrophoric, which can be used to form cure on demand polymer systems or can be used in adhesive systems which cure on demand. What is further needed are adhesive systems which are capable of bonding to low surface energy substrates, and initiator systems which facilitate such bonding. In addition to such needs, the complexes need to be thermally stable, that is do not disassociate at, or near, ambient temperatures and thereby initiate polymerization before desired. What are further needed are polymer compositions and adhesive systems which are thermally stable at, or near, ambient temperatures and which will undergo polymerization when the user desires. Further compositions that can be used in existing commercial equipment at mix ratios of 4:1 or less are needed. Compositions that have stability, strength and adhesion at elevated temperatures are desired.